The present invention relates to a streaming magnetic tape drive apparatus which writes or reads data while winding a magnetic tape at a constant speed and, more particularly, to an improvement of a repositioner, used in the streaming magnetic tape drive apparatus, for repositioning a magnetic tape.
A conventional magnetic tape drive apparatus used as an external storage apparatus for a computer adopts a start/stop method in which a magnetic tape is temporarily stopped at an inter block gap (to be referred to as an IBG hereinafter) between adjacent data blocks for each data block during tape travel. This method is achieved by controlling tape travel by a capstan motor arranged in addition to a reel motor.
However, in recent years, another method (i.e., a streaming method) has become popular, in which the capstan motor is omitted, in view of high-speed tape travel or reduced cost, and a magnetic tape is continuously wound without being stopped at each IBG under the direct control of the reel motor. However, in a streaming magnetic tape drive apparatus in which a magnetic tape is controlled directly by the reel motor instead of a capstan motor, if the magnetic tape is required to stop at a certain IBG (because, for instance, a command from a host computer is delayed, which prevents the magnetic tape from maintaining its streaming travel, or written data including an error) and is made to stop there, the magnetic tape cannot stop at the IBG but will stop in the next data block. This is due to the inertia of its reels and the small length of the IBG (normally 0.6 inch). Therefore, in order to reposition the tape, an operation for rewinding the tape from the stopped point to an intermediate point of a designated IBG is required.
The repositioning operation of a repositioner used in the conventional streaming magnetic tape drive apparatus is performed based on tape position data from a tachometer and the like provided to a tape travel system. More specifically, assume that a stop signal is supplied to a reel motor when the tape has just reached the intermediate point of an Nth IBG passing an Nth data block while the tape-streaming travels at constant speed in the forward direction. In this case, the tape cannot be immediately stopped due to the inertia of its reels, and the tape speed is gradually reduced and the tape is stopped at a point past the Nth IBG. A tape travel distance from the point at which a tape travel stop instruction has been generated to the point at which the tape is actually stopped is measured by, e.g., a tachometer. The tachometer comprises, e.g., a light-emitting element, a light-receiving element, and a rotating member, which is arranged between these elements, has a slit, and is in frictional contact with the tape. The tachometer generates a pulse each time light from the light-emitting element is received by the light-receiving element upon rotation of the rotating member. The output pulses are supplied to, e.g., an up-down counter, and are counted up.
The tape is then wound in the reverse direction by the reel motor. In this case, the tape cannot travel immediately at constant speed, due to the inertia of the reel, but its speed is gradually increased to reach a constant speed. At a predetermined period of time after the tape has reached the constant speed, a stop signal is supplied to the reel motor. The tape is gradually decelerated due to the inertia of the reel and is then stopped. During the reverse tape travel, the output pulses from the tachometer are counted down by the up-down counter. Therefore, the count value of the up-down counter exhibits a negative value corresponding to the absolute value of the difference between the initial up-count value and the down-count value.
The tape is then wound in the forward direction. The tape speed is gradually increased until it reaches the constant speed. In response to the tape travel, the output pulses from the tachometer are counted up. The point at which the tape has reached the constant speed and the negative count value of the up-down counter becomes zero is considered to coincide with an original point at which the tape travel stop instruction was initially generated. Therefore, the repositioning operation is completed at this point, and the tape forward travel is continued. Then, read/write operations for the next and thereafter data blocks are made.
However, in the repositioner of the conventional streaming magnetic tape drive apparatus which performs the above-mentioned operation, slippage between the tape and the rotating member of the tachometer and count errors of the output pulses of the tachometer are directly reflected as tape position detection errors of the tachometer. Therefore, the tape position at which the up-down counter starts counting does not always coincide with the tape position at which the negative count value of the counter is increased to zero since errors are accumulated over a long distance. Therefore, the tape position at which the negative count value of the counter is increased to zero cannot coincide with the tape position at which the counting operation starts, i.e., the Nth IBG, and often enters the Nth or (N+1)th data block. For this reason, adjacent correct data blocks are sometimes erroneously erased or a read operation is started midway along an adjacent data block.